This invention relates to an expandable polymer composition and a process for preparing lightly crosslinked, extruded, closed-cell foamed polymer articles from that composition. It particularly pertains to expandable ethylenic or styrenic polymer compositions containing a reversible crosslinking system which permits alcohol control of the degree of crosslinking of the polymer prior to extrusion foaming.
It is well known to make closed-cell polymer resin foams by the process of extrusion foaming wherein a normally solid thermoplastic polymer resin is heat-plastified and mixed under pressure with a volatile material to form a flowable gel which is then passed through a shaping orifice or die opening into a zone of lower pressure. Upon the release of pressure, the volatile constituent of the gel vaporizes, forming a gas phase cellular structure in the gel which cools to a corresponding cellular foamed solid resin. Desirably, the resulting gas cells are substantially uniform in size, uniformly distributed through the foam body, and closed, i.e., separated from each other by membrane walls of resin.
It is also known that the use of relatively lightly to moderately crosslinked polymers generally improves the quality of foamed polymer articles.
In addition, lightly crosslinking in some instances make possible foaming of polymer foams which otherwise cannot easily be produced. Some polymers such as linear polyethylenes are difficult to foam by extrusion. It is generally believed that poor melt strength together with a sharp change in melt viscosity near the transition temperature makes extrusion foaming of linear polyolefins difficult. Since light crosslinking increases polymer viscosity and thus broadens the range of foaming temperature, crosslinking would also be desirable from this standpoint.
However, a crosslinked polymer is difficult to extrude. As such, past practices have ordinarily not involved crosslinking during normal thermoplastic fabrication processing procedures such as production of extruded foamed polymer articles. As a result, most research works have been directed to production of a crosslinked polymer composition expandable during post-extrusion secondary foaming. Recently, however, advances have been made in overcoming some of the problems involved.
For example, Corbett U.S. Pat. No. 4,454,086 (assigned to the assignee of the present invention) discloses making crosslinked styrene polymer foams by an extrusion process. In Corbett a styrene/acrylic acid copolymer is lightly crosslinked in the foam extrusion line with a multi-functional epoxy resin. Since covalent bonds formed by the acid/epoxy reactions are not reversible, the scheme calls for a close control of epoxy level or the reaction rate.
In addition, silane and peroxide crosslinkers have been used to crosslink polyolefin and polystyrene foams which may be produced by using an extrusion foaming machine. U.S. Pat. Nos. 4,446,254; 4,421,867; 4,351,910, and 4,252,906, amongst others, fall into this category. Sugitani Patent No. 4,351,910, for example, proposes improving the heat resistance of a polystyrene foam by introducing an organosilane compound into a styrene series resin. The silane structure is chemically bonded to the molecular structure of the styrene series resin by addition polymerization, by graft polymerization or by free radicals. The degree of crosslinking is disclosed as being temperature dependent. As such, Sugitani states that crosslinking can be delayed by low temperature processing since it only proceeds gradually at temperatures below 100.degree. C.
It is also known that crosslinking can be delayed by swelling the polymer so as to permit working in crosslinking agents at temperatures below the starting point of the used crosslinking agents. Thus, Slogburg U.S. Pat. No. 3,452,123 discloses adding an organic solvent to swell an ethylene polymer and then admixing therein, at a temperature below the starting point of the used crosslinking agent, the propellant and the crosslinking agent. Extrusion of the resulting mass is carried out at temperatures above the softening point of the swelled compound. This system is said to result in delay of the crosslinking so that it, preferably, occurs in the extrusion die.
Still, the delayed crosslinking system of Slogburg, like the crosslinking and foaming systems of the others mentioned, is not reversible and therefore requires rather careful temperature and processing controls. The need exists therefore for improved means for controlling the degree of crosslinking of an expandable polymer prior to extrusion foaming.